1. Technical Field of the Invention
The present invention relates to a terahertz beam scanning apparatus and method for scanning a terahertz beam.
2. Description of the Related Art
Terahertz waves are electromagnetic waves that lie between radio waves and infrared radiation and are 0.3 THz to 10 THz in frequency (30 μm to 1 mm in wavelength). Such terahertz waves have penetrability to various substances such as paper and plastics and a proper spatial resolution, and are expected to serve as a practical tool for radioscopic imaging of various objects. Previous studies have demonstrated usefulness of terahertz waves.
However, in conventional radioscopic imaging that uses terahertz waves, a sample is irradiated with a focused beam of a terahertz wave (hereafter a terahertz wave having directionality is referred to as a “terahertz beam”), where the terahertz beam is scanned by mechanically moving the sample itself. A scanning speed of the terahertz beam is limited by this mechanical operation. Thus, there is a problem that radioscopic imaging requires a long time.
In view of this, to accelerate radioscopic imaging that uses terahertz waves, means for scanning the terahertz beam while fixing the sample have already been proposed in Patent Documents 1 to 3.
In Patent Document 1, a galvano-scanner mirror is used in laser beam scanning.
In Patent Document 2, the terahertz beam is scanned using a phased array antenna.
In the invention of Patent Document 2, when generating the terahertz wave by difference frequency mixing using two laser beams that differ in wavelength, a wave front of the terahertz beam is indirectly tilted by shifting a phase difference of the laser beams through the use of an optical phase shifter for each array element, thereby scanning the terahertz beam.
In Patent Document 3, the beam is scanned by controlling a whole array phase altogether through the use of one spatial optical phase modulator, instead of a lot of optical phase shifters in Patent Document 2.
[Patent Document 1]
Japanese Patent Application Laid-Open No. 2006-224142, “Laser Scanner and Laser Marking Apparatus, and Laser Marking Method”
[Patent Document 2]
Japanese Patent Application Laid-Open No. 2007-103997, “Electromagnetic Wave Emission Device”
[Patent Document 3]
Japanese Patent Application Laid-Open No. 2008-5205, “Optical Control Type Phased Array Radar Apparatus”
The galvano-scanner mirror in Patent Document 1 has a problem of a slow operation speed of 1 kHz (1000 times per second) at the maximum, because the mirror is mechanically operated.
In Patent Document 2, the optical phase shifter is used for each array element. In the case of increasing an array scale, there is a problem of increases in size and cost of the entire device because a lot of phase shifters corresponding to the number of elements are needed.
Besides, though research has been conducted on an electrical phase shifter using a liquid crystal in a terahertz band, an amount of phase shift has so far been insufficient, making it difficult to scan the terahertz beam at high speed over a wide angle.
In Patent Document 3, one spatial optical modulator is employed instead of a lot of phase shifters. However, the modulator has a slow response speed and so is unsuitable for high-speed terahertz beam scanning.